Transgenic mouse models of SODI-linked ALS show a number of similar phenotypes. First, all mice that express the mutant protein at high levels [>3 fold over endogenous) develop a progressive paralytic disease. Second, in the interval between the onset of symptoms and human endpoint, spinal cord tissues accumulate large, detergent-insoluble, aggregates of mutant SODl. Third, prior to the onset of symptoms, a number of pathologic abnormalities appear in spinal cord, including loss of muscle innervation, astrogliosis, and pathologic changes in motor neuron morphology. In the prior award period, we uncovered a link between the inherent ability of mutant SODl to form large, sedimentable, aggregates and the rate at which disease progresses in humans. For example the A4V mutation in SODl is associated with short duration disease and is highly prone to aggregate. By contrast the H46R mutation in SODl is associated with disease of long duration (>15 years) and is much less prone to form aggregates. In the present application, we propose 4 Aims that will clarify the role of mutant SODl aggregation, and/or multimerization, in that pathogenesis of ALS. Aim 1 will directly follow up on studies of the first award period to further investigate the association between aggregation of mutant SODl and disease progression. We will determine whether all disease-associated mutations in SODl cause protein aggregation and use a multifactoral approach to determine the relationship between aggregation and disease progression. Aim 2 will directly test the role of mutant SODl aggregation in disease progression by altering mutant protein aggregation in transgenic SODl mouse models. Multiple approaches will be used to manipulate aggregation of mutant SODl in mice. Aim 3 will determine define the relationships between mutant SODl multimerization and the evolution of disease. Aim 4 will seek to determine the mechanism by which co-expression of wild-type human SODl in mutant mice hastens the onset of disease. High level expression of wild-type SODl augments a toxicity that hastens the onset of disease and may affect the rate of disease progression. At the conclusion of these studies, we will have clarified the nature of SODl proteins that induce early disease phenotypes and determined the role of mutant SODl multimerization in disease progression.